Ground potential compensator



Sept- 8, 1953 c. L. I duDoN 2,651,773

GROUND POTENTIAL cQMENsAToR Filed May 7, 1952 3 Sheets-Sheet l ATTORNEY Sept. 8, 1953 c. 1.-. I oUDoN 2,651,773

GROUND POTENTIAL COMPENSATOR v Fild May 7. 1952 5 sheets-sheet 2 INVENTOR. v CI Ifford L Loudon ATTORNEY NGI Sept. 8, 1953 c. L.. LouDoN GROUND POTENTIAL COMPENSATOR 5 Sheets-Sheet 5 Filed May 7, 1952 ciigford L. IVNS Q/ @7% ATTORNEY Patented Sept. 8, 1953 GROUND POTENTIAL COMPENSATOR Clifford Lee Loudon, La Jolla, Calif., assignor to Holmes Electric Protective Company, New York, N. Y., a, corporation of New York Application May 7, 1952, Serial No. 286,647

7 Claims.

This invention relates to ground potential compensators which make possible the use of the earth as a conductor in circuits which normally carry a constant current. More particularly it has to do with a device which detects any change in potential between the two grounded points which may be widely separated in the circuit and instantly impresses on the circuit an equal but opposite potential.

When parts of a circuit are required to be far apart, as for instance in electrical burglar alarm systems where the signal producing equipment on the protected premises is several miles from the central office, the economical use of cable conductors makes it attractive to employ the earth as a conductor. In this Way a single cable conductor can be used to connect the signal producing equipment to the central oilice rather than a full metallic circuit.

In the past, however, it has not been possible to fully utilize this advantage because of the presence of earth currents caused by the relatively enormous powers used in urban areas by traction lines and industrial plants. These earth currents result in the development of appreciable stray voltages over distances of several miles, a condition which is particularly troublesome in that these voltages are continuously and erratically varying in polarity and magnitude. As a result those electrical systems which function on the principle of using a variation in their normally constant current to transmit a signal have not-heretofore'been able to utilize the earth as a part of the circuit because these stray potentials will themselves vary the current and produce a false alarm.

It is an object of the present invention to proy vide a device which will rapidly and precisely compensate for all values of ground potential by impressiong on the circuit an equal and opposite potential.

The best mode in which it has been contemplated applying the principles of my invention Fig. 2 is a block diagram of the ground potential compensator itseli; and,

Fig. 3 is a more detailed schematic diagram of the ground potential compensator embodying the present invention.

Referring now more particularly to the drawings, Fig. l shows my novel ground potential compensator I0 arranged to provide compensation for a number of signalling circuits in an electrical burglar protection system. In each signalling circuit the signalling equipment I2 which is located on a subscribers premises I4 and Which is responsive to unauthorized intrusion is connected to a central office I 6 by a single cable conductor I8 which carries a current supplied by a central office battery 20. The signalling circuit is completed by an earth return. Normally the current in each signalling circuit is constant. A change in this current such as would be produced by disturbing the subscribers signalling equipment I2 gives an alarm in the central onice equipment 2l. My novel ground potential compensator I0 prevents changes in ground potential from affecting this current and producing false alarms.

In order that one ground potential compensator may provide compensation for a group of signalling circuits, as shown, each such circuit must be subject to the same ground potential influences. This is made a certainty by grounding the individual signalling circuits at a common point 22, near the several protected premises. Usually this will require the use of an additional cable conductor 26 between each subscribers signalling equipment I2 and the common ground A22, but the length of each such conductor 26 is always a small proportion of the distance from a subscribers signalling equipment I2 to the central olice I6, and a considerable saving in cable is accordingly realized. The earth return is, of course, completed by grounding one side of the central office battery 2U at point 28 to the ground.

Changes in the magnitude and polarity of the ground potential in the earth return are reilected in changes in voltage drop across a voltage divider 30 connected between the local cnice and central ofiice grounds 22 and 28, respectively. This ground potential detecting circuit Will be referred to as the pilot cable 32, and values of potential at selected points on the voltage divider 30 therein constitute the input control voltages for my novel compensator.

Having provided a common ground path for a group of signalling circuits, thus making them equally aifected by the same ground potentials,

3 and having provided a means for instantaneously and continuously determining the magnitude and polarity of this potential, it is possible to use electronic apparatus to control the release of power to exactly counteract the ground potential.

Since it is customary to use storage batteries to provide power for the signalling circuits, the power furnished for compensation must be direct current from a low resistance source. It is not practical to rectify and filter power from the alternating current mains and then feed Lit directly into the alarm circuits, because the required smoothing filter would introduce toomuch electrical delay into the system.

In this system low frequency power from the alternating current mains is rectified, filtered, and then converted to high frequency `alternating power. Then, under the control of the ground potential pilot voltage, this high frequency power is amplied and rectified and fed into the signalling circuits withoutthe need for a smoothing lter, the ripple frequency being too high to interfere with the normal operation of the signalling circuits.

The general method by `which this multiple conversion is performed in my preferred embodiment is shown in Fig. 2. Power supply 34, in conjunction with high frequency generator 35, converts nominally 110 volt, 60 cycle energy into high frequency energy suitable for use in this system. This high frequency energy is fed to the control tube stages 38 and 38' where it is introduced into the positive and negative sections respectively of the compensator in proportion to the instantaneously measured ground potentials which are also applied to the same control stages. The output from each control stage is subsequently amplified by intermediate amplifler stages 40 and 40 and power amplifier stages i2 and 42 and delivered to rectifier stages 44 and 44. Power supply 4E energizes a phase inverter stage 47 and all other stages except the rectifier stages 44 and 44 with high voltage direct current converted from alternating current energy similar to that fed to power supply 34. After amplification and rectification the output from each section of the compensator is so injected into a common power feed conductor iii-3248" for the several ,alarm circuits that the effect of the ground potential of the instant on the alarm circuits is cancelled out by opposition.

Fig. 3 shows the preferred form of my `novel ground potential compensator in somewhat greater detail. For .ground potentials at ground point 22 which are positive with respect to central cnice ground point 2B, as measured by voltrneter 50 in pilot conductor 32, a resistance coupled ampliiier vacuum tube 52 is used with its third grid 52al connected through a resistance ft to the junction of potentiometer ,30a `and resistance 3th on the voltage divider 30. Capacitor 56 connected to this input circuit between grid 52d and resistance 54 `by-passes to ground any A. C. input components which are high enough in frequency to be harmless to the alarm system, thus preventing useless expenditure of compensating energy. This capacitor is also helpful in preventing parasitic oscillations. Grid E20. is biased negatively with respect to the cathode 52h by connecting the cathode to the junction of resistances 58h and 58e on another voltage divider 58 which is interposedlbetween ground 26 and a tap 46a on power supply 46. The grid 52a is also biased negatively with respect to the second grid 52C by connecting the latter to the junction of resistances 53a and 58h on voltage divider 5B. These connections cause the transconductance of tube 52 to Vary in proportion to positive ground potentials. A small audible frequency voltage of constant root means square voltage from high frequency generator St is applied to the first grid 52d of tube 52 by connecting this grid to the junction of resistances 60a, 68h and 60e on a third voltage divider 60. In additionthis `grid 52d received a fixed negative D. C. bias from the junction of resistances 58e, 58d and 58e on voltage divider 58. A resistance 62 .is interposed in lthe circuit between this junction and grid 52d. With proper choice of electrode voltages, the A. C. output of tube 52 appearing across a potentiometer 64 `in the plate circuit will be practically zero for negative ground potentials and will vary in a linear relation with positive potentials up `to specified values. The D..C.gplate supply is obtained at tap 4Gb oni-power supply and capacitor 66, connected to the junction of potentiometer and a resistance &8 in the plate circuit, by-passes the A. C. output of tube 52 around the power supply. For tube 52 l prefer to use `a type 6L? -vacuum tube `with its metal casing grounded, as showngbut `other, tubes may be employed with circuit modifications according to their characteristics.

For negative ground potentials `an inverter tube and ya control tube are used. Since the ktransconductance of a vacuum tube becomes lgreater as the control electrodes become less negative or more positive a vacuum tube 10 is used to convert negative ground potential 4increments into positive increments. .This amounts to phase 'inversion. A portion of the ground potential is applied to the control 4grids 10a of tube 10, which is also arranged as a resistance coupled amplifier, by connecting these grids to the junction of resistances 30h and 30a on voltage divider 30. A resistance 12 is locatedin this input circuit. With a suitable value of fixed grid-cathode bias, ob-

' tained by .connecting the cathode to the junction of resistances `Uhr and 'Mb on voltage divider 14, negative increments of ground potential will cause decreases in the D. C. current to the plates 10c which draw current througha common load .resistor l5 of high resistance. `A decrease in this plate current causes a decrease .in voltage drop across the load resistor 16 and consequently van increase in plate-cathode potential. The plate circuit supply voltage for tube '1.0 is also obtained from tap `46a on power supply 4.6. For this tube 'i0 I prefer -to use a type SSN? vacuum tube with the Vtwo sections connected in parallel to obtain a large output current -for a given grid voltage., but again other tubes `may be used with appropriate circuit changes.

By connecting the screengrid a of a resistance coupled amplifier-Sil to the plates 10c of tube 10 the transconductance of tube .8.0 is caused to increase as the ground potential becomes more negative. :Capacitor 82 functions similarly to capacitor 5S.

When the ground potential becomes positive. the effective voltage on the grids 10a of tube 1.0 assumes a low negative or positive value,.,depend ing on the magnitude of the ground potential. Under these conditions the grid voltage vs. plate current characteristic of tube 'Ill flattensoff and `thescreen-grid voltage and transconductance of tube Si] become very low and static. As with tube 52, the small audible frequency voltage from high frequency generator 36 is introduced-into tube 80 by connecting the rst grid 80h thereof to the junction of resistances 60h, 60e and 68d on voltage divider 60. Condenser 84 prevents any D. C. voltage from voltage divider 58 from reaching tube 80. Cathode 88o of tube 80 receives its bias from the junction of resistances 86a, 86h and 88e on a voltage divider 86. The plate circuit supply voltage for tube 80 is obtained from tap 40h on power supply 46'. With proper choice of electrode voltages the A. C. output of tube 80 appearing across a potentiometer 88 becomes practically zero for positive ground potentials and varies in a linear relation with negative ground potentials up to a specified value. Capacitor 90, connected to the junction of potentiometer 88 and a resistance 92 in the plate circuit by-passes the A. C. output of tube 80 around the power supply.

Following these control tube stages the two branches of the compensator are similar. In each branch voltage amplification is obtained in a triode resistance-capacitance coupled amplifier stage in which I prefer to use a type 6.15 vacuum tube 94. Thereafter low level power amplification is obtained in a triode connected transformer coupled stage, in which I prefer to use a type 6F6 vacuum tube 86, and high eflicienoy amplification is obtained in a beam power stage in which I prefer to use two 807 vacuum tubes 98 arranged in a push-pull circuit, as shown. The outputs of the final ampliiication stages are converted into direct current by selenium rectiiiers |08 and |08. terminating in resistors |02 and |04 respectively of low resistance. a substantially constant load to the beam power tubes 98 at the cost of some dissipated power, and they are connected in series with each other and in series with the power feed 48--48-48 leading from the central office battery 20 to the group of signalling circuits, with developed p0- larities opposing. The amplification in each branch is adjusted so that the voltage across the two output resistors |02 and |04 is equal at every instant to the ground potential.

The features of my novel ground potential compensator will be fully understood from the above description, but some additional discussion including values for the various voltages, frequencies and resistances will be helpful. My novel device works well with these values but they are, of course, merely illustrative.

Referring again to Fig. 3, only one vacuum tube is required for the control of energy for positive ground potentials. Both the rst and third grids 52d and 52a, respectively, of this tube are control grids affording linear control effects with small voltage requirements. The cathode 521) is connected to the junction of resistances 58D (2700 ohms) and 58e (240 ohms) on voltage divider 58 making it positive with respect to ground since 120 volts are applied to this voltage divider from tap 46a on the power supply 46. The third grid 52a. is connected to ground through resistance 54 (47,000 ohms) and resistances 38o (2000 ohms) and 38C (510 ohms) in voltage divider 30, making this grid about 8.5 volts negative with respect to vthe cathode 52h in the absence of any pilot circuit voltage. The rst Vgrid 52d is returned through resistance B2 (47,000 ohms) to the junction of resistances 58e, 58d (160 ohms) and 58e (750 ohms) on Voltage divider 58, making it about 5.5 volts negative with respect to the cathode 52h. The second grid 52o is connected to the 'junction or" resistances `58a (2200 ohms) and 58h These resistors help to present 6. on voltage divider 58. A 5000 cycle signal of 2.0 R. M. S. volts is maintained on grid 52d from the junction of resistances 60a (220 ohms), 60D (1000 ohms) and 60o (5100 ohms) on voltage divider 60.

VIn the absence of any ground potential or in the presence of a negative one1 the effective negative bias on the third grid 52a is high enough to reduce the transconductance and amplification of tube 52 nearly to zero. Positive ground potentials reduce the effective negative bias on the third grid 52a causing the amplication to increase. The output oi this tube is transferred to the succeeding ampliiier tube 94 through gain control potentiometer 64 (50,000 ohms). The D. C. plate voltage is obtained from tap 45h (150 volts) on power supply 45 and is drawn through resistance 68 (33,000 ohms) in series with potentiometer 64.

In the negative ground potential compensator section the two triodes of tube l0 are connected in parallel to provide high mutual conductance with a medium amplification factor. The voltage drop across resistance 14a (270 ohms) of voltage divider 14 provides a constant grid to cathode bias of -lA volts. The plate loadresistor 16 is of high value (820,000 ohms) so that a small current through it will cause a large voltage drop across it. The plates 10c are approximately 15.0 Volts above ground with no ground potential applied to grids 70a. These grids are connected to the junction of potentiometer 30a (5000 ohms) and resistance 30h on voltage divider 30 through resistance l2 (1,000,000 ohms).

The cathode e of tube 80 is connected to the junction of resistances 80a (270 ohms), 88h (390 ohms) and 88o (8200 ohms) which is 2.0 volts positive to ground, making the first grid 80h negative to the cathode by 2.0 volts. A 5000 cycle signal of .9 R. lVI. S. volts is applied to this rst grid 80h from the junction of resistances 00h, 60e and 50d (1000 ohms) on voltage divider 00. This signal passes through condenser 84 (.10 microfarads).

In the absence of any ground potential or in the presence of a positive one, the plate current of tube 10 is high and the plate to cathode voltage is 15.0 volts or less. Since the sceen grid 80a of tube 80 is also 15.0 volts or less the transconductance of tube 88 is very low and very little audible frequency voltage appears in its output circuit. The high resistance l2 in the grid circuit of tube 10 prevents large grid currents from owing as a result of high positive ground potentials, thereby avoiding any possibility of up- -setting the resistance relations in the voltage divider 30 of pilot circuit 32. With negative ground potentials, the grids 70a of tube i0 oecome more negative, the plate current decreases and the voltage on the screen-grid 80a of tube 80 increases. As a result the R. M. S. output voltage of tube 80 rises in proportion to the values of the total ground potential voltage.

IThe circuits associated with tubes 94, 9G, and 98 are quite conventional. The two amplifier tubes 94 function as ordinary resistance-capacitance coupled amplifiers to provide voltage gain. The two tubes 96 are connected as triodes to obtain low plate resistance. They require step down plate transformers |06 to match the low input impedance of tubes 08 which are operated as push-pull class AB-2 amplifiers. Due to the fact that high efficiency vacuum tube amplifiers .require a constant load impedance for stable performance, the power ampliers 98 terminate 7 in xedloadiresistors |02 and 10.4,. This resistance is made low compared to the resultant parallel .resistance of all the signalling `circuits connected to it. Vacuum tubes .also have 1a high internal impedance. The impedance "transformation required here causes a voltage fstep down so that some voltage amplification is required to offset the reduction.

The polarity of the output connections for the full wave bridge `rectiiiers i100 :and |00' will depend `upon which pole fof the `central -ofllce .battery 20 is grounded at point 28. The connections indicated in Fig. 3 are correct for the Acase where the positive terminal of the battery is grounded. With these connections it can be -seen ithat the normal current of the power vfeed lcan "pass through both branches :of the Vrectifier 100' controlled by tube 10, 4whileboth .branches of rectier controlled by tube 52 are non-conducting for normal line current. Because resistance -|04a (l5 ohms) and |041) (15 ohms) of resistor .|04 are thus shunted bythe rectifier, their effective resistance value is set at about one fourth `of the .lowest effective `value lof signalling circuit resistance to be connected with it. In vth-is way variations in the number of lines connected will have negligible effect on the impedance match between the power `ampliiier tubes 98 and their load. At the same time, the power output required of the amplifier ytubes `93 is kept .ata minimum.

In the case of resistances I02a `(5 ohms) and |021) (5 ohms) of resistor |02 the value of 'resistance is made smaller since the voltage-,drop due to the power `feed current through them lowers Athe effective Voltage of battery 20. In the .apparatus found satisfactory the maximum power feed current is one ampere causing Aa voltage `drop across resistor |02 of `2.5 volts. However, the control characteristics of the circuit are such that enough 5000 cycle signal is allowed to pass, with zero ground potential, tto develop 12.5 volts'in the negative compensating rectifier which balances out this voltage drop. Due to the 'lower ,amplier load resistance of resistor l0-2, the power output capacity of Athis section :must :be three times that of .section controlled by tube .for an equal voltage output.

Although the device just described provides accurate compensation :for ground potentials .up to better than 10.0 peak volts, ,sufficient for Aany condition so far encountered, only reasonable modiiications are required to laccommodate much higher values. Thus, if `it were desired to triple this range, an additional resistance 'twice ithe combined value of .,30a, 30h and 30C involtage divider 30 can be connected ,in the .pilot y.circuit line 32 between potentiometer 30a and ground point 2.2. Then `a ground potential of 30.0 volts would have the same effect on 'the circuit as 10.0 volts do in the embodiment of the invention disclosed herein, and lall intermediate values would give proportionate correction. By adding additional amplifier stages with voltage gains of 3 and power capacities :9 times as great, the compensation would be complete.

I claim:

1, A ground potential compensator forran electrical circuit having `a signal conductor which carries a current between `two ground points and in which signal conductor said `current is afected by a difference .in ground potential between said points, said ground potential compensator comprising .a pilot conductor in parallel with said signal conductor Iand having a res'istm therein, a power source, Acontrol tube `means drawing power vfrom said source .and having a power output kproportional to a positive ground potential ted theretofrom said resistor, said control tube means being unresponsive to a negative ground potential fed thereto, inverter tube means for converting Va negative ground potential fed thereto :from 4said resistor to a positive potential andiunresponsive to a positive ground potential so fed, additional Vcontrol tube means drawing additionazl power :from saidpower source and having apower output lproportinal to the output of said inverter tube'means `fed thereto, separate amplier means ,for amplifying the output of saidcontrol tube means and said additional control tube means respectively and having their respective outputsconnected tosaid electrical circuit so as 'to .compensate for vthe reiect therein of said ground potential.

12. grou-nd .potential compensator for an 'elec- .trical circuit khaving a signal conductor which caniesa vcurrent between Itwo ground points and -in which signal conductor said current is affected by fa iditference in ,ground potential -between said points., :said ground lpotential compensator comprising a pilot-conductor in parallel with said signal conductor and having a resistor therein, lcontrol tube -L-means, a :voltage source providing .a :voltagefwhichis modulated by said control tube meansin proportion to positive ground potentials `ed thereto from said resistor, said vcontrol tube means being .inactive when :a negative ground potential is .fed thereto, inverter tube means for 4corxverting'fa 'negativegground potential fed there- :torrone Asaid resistor pinto apostive potential and inaotivewhen `a positive ground potential vis fed thereto, :additional control tube means which `modulates additional voltage vfrom said voltage source in proportion .to the output .of said inverter tube means .fed-thereto, separate amplifier meansor amplifying the output 4of saidcontrol `tube means and said additional control tube :means respectively, and `having their respective `outputs connected to ysaid electrical circuit so as to oppose and cancel .therein the elect of said ground potential.

,A `grou-nd potential compensator for an electrical circuit having a metallic signal conductor which carries a normally constant direct current between `a nrst ground point and a second ground -point'widely separated therefrom and in which signal conductor'saidcurrent is affected by a dif- .erence in ground potential .between said points. -said ground potential `compensator comprising .a pilot conductor connected between said ground :points and having a voltage divider therein Vto detect instantaneously the magnitude and polar- .ity of said difference in ground potential, an :a1- ternating power source, control vacuum tube means ,receiving as an input the potential at a jpoin-t onsa-id voltage divider, drawing power from said source, having 4a power output proportional to the amount by which a potential at the first said ground point is positive with respect to the corresponding potential at the second `said .ground point, and -being inactive when a poten- .talat the iirst Said ground point is negative with respect `to Vthe corresponding potential at the second said `ground ipoint, amplier vacuum tube means for :amplifying the power output of said .control vacuum tube means, rectiiier means for rectifying the output of said `amplifier vacuum tube means and having its output connected to said electrical circuit so as to compensate the effect on-said current flowing therein of a potential at the first said ground point which is positive with respect to the corresponding potential at the second said ground point, inverter vacuum tube means receiving as an input the potential at a point on said voltage divider, having a phase inverted outputl proportional to the amount by which a potential at the iirst said ground point is negative with respect to the corresponding potential at the second said ground point, and being inactive when a potential at the rst said ground point is positive with respect to the corresponding potential at the second said ground point, additional control Vacuum tube means receiving as input the output of said inverter vacuum tube means, drawing additional power from said source, and having a power output proportional to the amount by which a potential at the first said ground point is negative with respect to the corresponding potential at the second said ground point, additional amplifier vacuum tube means for amplifying the power output of said additional control tube means, additional rectifier means for rectifying the output of said additional amplier vacuum tube means and having its output connected to said electrical circuitv so as to compensate the effect on said current iiowing therein of a potential at the first said ground point which is negative with respect to the corresponding potential at the second said ground point.

4. A potential compensator for an electrical circuit having a metallic signal conductor which carries a normally constant direct current between a first ground point and a second ground point Widely separated therefrom and in which signal conductor said current is affected by a difference in ground potential between said points, said ground potential compensator comprising a pilot conductor connected between said ground points and having a voltage divider` therein to detect instantaneously the magnitude and polarity of said difference in ground potential, an alternating current voltage source, control vacuum tube means receiving as input the potential at a point on said voltage divider, modulating a voltage applied thereto from said source in proportion to the amount by which a potential at the first said ground point is positive with respect to the corresponding potential at the second said ground point, and being inactive when a potential at the first said ground point is negative with respect to the corresponding potential at the second said ground point, amplifier vacuum tube means for amplifying the voltage output of said control Vacuum tube means, rectifier means for rectifying the output of said amplifier vacuum tube means and having its output connected to said electrical circuit so as to oppose and cancel the effect on the current flowing therein of a potential at the iirst said ground point which is positive with respect to the corresponding potential at the second said ground point, inverter vacuum tube means receiving as an input the potential at a point on said voltage divider, having a phase inverter output proportional to the amount by which a potential at the first said ground point is negative with respect to the corresponding potential at the second said ground point and being inactive when a potential at the first said ground point is positive with respect to the corresponding potential at the second said ground point, additional control vacuum tube means receiving as input the output of said inverter vacuum tube means and modulating a voltage applied thereto from said source in propor- 10 tion to the amount by which a potential at the iirst said ground point is negative with respect to the corresponding potential at the second said yground point, additional ampliiier vacuum tube for amplifying the output voltage of said additional control tube means, additional rectifier means for rectifying the output of said additional amplifier vacuum tube means and having its output connected to said electrical circuit so as to oppose and. cancel the effect on said current owing therein of a potential at the iirst said ground point which is negative with respect to the corresponding potential at the second said ground point.

5. A ground potential compensator for an electrical alarm circuit connecting a plurality of local signaling equipments with a central oflice, said circuit having signal conductors extending from a common centro-l oiiice ground point to said local equipments and thence to a common ground point near said equipments and remote from the central oflice, each said signal conductor normally carrying a constant direct current which is aiected by a difference in ground potential between said ground points, said compensator comprising a pilot conductor connected between said ground points and having a voltage divider therein so arranged that a diiference in potential between said ground points causes a corresponding voltage drop between selected points on said voltage divider, a high frequency alternating voltage source, control vacuum tube means having a plate power supply, having one of its grids connected to a point on said voltage divider, having a second grid connected to said alternating voltage source and having said second grid and remaining elements so biased with .respect to said first mentioned grid that power drawn from said plate power supply by said control vacuum tube means appears in the output thereof as alternating Voltage proportional to the amount by which a potential at said local ground point is positive with respect to the corresponding potential at said central office ground point, said biased relation also causing said control vacuum tube to be inactive when a potential at said local ground point is negative with respect to the corresponding potential at said central oiiice ground point, amplifier vacuum tube means .for amplifying the alternating power output of said control vacuum tube means, rectier means for converting the alternating power output 0f said ampliiier vacuum tube means to direct power at a voltage equal to the amount by which the potential at said local ground point is more positive than the corresponding potential at said central oiiice ground point, said rectiiier means having its output connected to said electrical alarm circuit so as to oppose and counterbalance the eiiect on said current flowing in said signal conductors of a potential at said local ground point which is positive with respect to the corresponding potential at said central oflice ground point, phase inverter vacuum tube means having a plate power supply, having a grid connected to a point on said voltage divider and having its other elements so biased with respect to said grid that a negative potential occurring on said grid as a result of said local ground point being more negative than said central olice ground point causes the plate of said phase inverter vacuum tube to be proportionately more positive, and the last said biased relation also casing said phase inverter vacuum tube means to be inactive when acume la potential at said local 'groundipoint `is positive with 'respect to "the Icorresponding potential simultaneously A4occurring iat vsaid central oiiice ground point, additional "control vacuum Vtube means 'havinga 'plate power'supplm-having" one of its grids connected to"the;plate"of 'saidrphaseinverter vacuum tube'means having-af-secondigrid connected to :said alternating volta-ge source 'and having said 'second grid'and'remaining elements so biasedWithgrespectfto its-'saidiirst` mentioned grid that y'power drawn V`from :said plate 'power supply by said"additionalcontrol vacuumftube 4means appears in'ithe "outputthereof as alternating voltage proportional to the amount kay-which 4a potential at said-local ground point is negative with ,respect rto 'the ycorresponding potential 'at said central oiiice groiin'dpoint vadditional `am- `plier vacuunrtube `rneans for' amplifyingthe` alternating power output of'sa'i'd additionalfcontrol 'Vacuum tube means;additionalrectier means for converting the `alternating"power output of --said additional ampliiierv-acuum'tubemeans to direct power at a voltage equal tothe lamountiby which the ypotential at saidlocal 'ground point is more'negative than 'the corresponding potential -at Y'said central office 4groundpoin't,said ladditional rectier Vmeans yhaving itsoutput `connected to `said electrical alarm circuit #so fasfto oppose and counterbalance the effect on said current 'owing in `said signal `fconductorsof-a potential at 'said l`-local ground point which l'is vnegative with'respect to lthe corresponding'po- `tential at said-central oilicefgroundpoint.

6. A ground potential compensator for 4an electrical circuit having a -signal conductor which carries a current 'between two 'ground Vpoints and in `which signal lconductor f the Ycurrent is affected byafdifference groundpotential between sai'd'pointa said 'ground potential Acompensator ycomprising y-a pilot conductor in parallel `with the signal `conductor, a =power source, control amplifying'means` drawing power from the source and having-an 'output proportional to positive 'groundgpotentials vfed thereto from thepilot conductor, vsaid-control amplifying means being unresponsive tof-negativefground potentials fed'thereto, inverter `means'for-converting lnegative 4ground 'potentials "fed *thereto "points and `in which signal ^conductor T"saidrcurrent fis affected by'a. difference in Jgroundf'potential between said points, said :ground potential `compensator 'comprising =a :pilot :conductor in Yparallel vwith 'said -signal r`conductor, Ya :power vsource, control means drawing f" power "'f rom f said source fand'having anoutput proportional Eto "positive ground potentialsf ed #thereto ."fromsaid pilot conductor, saidk means l Alte'ing unresponsive nto a y'negative ground fpotential Pied thereto, polarity changing l--means "fon 'converting a negaf-tiveflground potential fed! thereto "from said. pilot conductor `to a i positive potential and unresponsive 'to-:a positive ground i potential vso 'fed,1addi Iiti-onal r control fmeans drawing additional :power from said power source and havingfan output proportional .to positive fpotentias fed thereto "from Ysaid polarity :changing means, .separate amplifier `means `for `amplifying `:the output :of -saidfcontrol means andrasaid :additional control means respectively `rand having ftheir #respective -outputs connected to'fsaidf electrical circuit :sofas to icompensa'te 'for the :effect "therein k of zlsaid ffgroundzzpotentials.

References VGitecl yin :the :nie il of this fpatent UNITED `-STATES EEfAI'ENI'S 

